DE3308584A1 - LASER TRANSMITTER RECEIVER WITH HETERODYNER DETECTION - Google Patents

Description

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LA SER-SEND EMBPAMGSORRIGHTUNG! WITH HETERODYNER DETECTION i

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The invention relates to a laser beam receiving device with heterodyner. Detection according to | the preamble of claim 1 - as well as the application of a ^ r such Device on a remote sensing system. j

In a known device of this type, the transmitted frequency of the laser transmitter is shifted slightly with respect to the frequency of the local oscillator, and the echo thrown back from the target is fed to a photoelectrophic detector which also receives the radiation supplied by the local oscillator. The detector then delivers signals which are modulated with a frequency corresponding to the difference between the transmission frequency | 3 and the frequency of the local oscillator. For this purpose, the laser transmitter delivers a continuous beam, which is divided into two parts by an optical lamella aligned obliquely to the transmission axis: A first part of the beam first passes through a frequency shift cell assigned to the modulator, so that pulses of radiation are obtained.

whose frequency is slightly relative to the frequency of the laser transmitter is shifted. These impulses then arrive optical system that aligns them with the target. The second part of the beam forms the radiation of the local Oscillator and is fed to the detector in order to be mixed with the echo radiation reflected from the target

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The known device described has a post

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part: it is namely difficult to prevent the formation of parasitic Reflections of the radiation pulses with a shifted frequency in the two surfaces of the optical system and on to shift the output side of the frequency shift angle

avoid. Some of these parasitic reflections pass through the cell in the opposite direction without a frequency shift and, after reflection at the mirrors of the laser cavity, are sent from the optical lamella to the detector. Hence, results a large level noise signal interfering with the operation of the device.

The object of the present invention is to remedy this disadvantage. Its object is a laser transceiver with heterodyne detection that contains;

- a laser transmitter that emits a continuous main radiation,

- an optical separator that removes part of the radiation I and the raw + · lets through,

- a low frequency modulator (frequency f-), which the from Separator receives the transmitted part of the main radiation and emits corresponding laser pulses,

- an optical system attached to the output of the modulator, which directs the laser pulses onto the target, with part of the Energy of these impulses is backscattered according to a reflection of the frequency f ..,

- a local oscillator that emits a laser reference radiation with a frequency f "φ f-i,

- An electro-optical receiver to receive the return radiation of the frequency f .. and the reference radiation of the frequency f _? and outputting an electrical response signal having the intermediate frequency ^ ^ 2 ~ - is \ uliert ^mod,

- and a circuit for processing the electrical signal,

characterized in that the local oscillator consists of a frequency shifter which is located between the separator and the receiver is and the part of the main radiation of the frequency f .. taken from the separator receives, this Organ supplies the reference radiation of frequency f ^.

In this device, the processing circuit in Row at the output of the receiver one after the other an intermediate

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frequency signal amplifier, an envelope detector and a Low frequency amplifier included, which is connected to the modulator and a synchronous detection at the low frequency ff. causes.

The present invention also has a remote sensing system to the article which contains a laser transceiver as defined above and is characterized in that it aside from that

- an additional laser transmitter, which supplies a continuous main radiation of a frequency f. ^ shifted with respect to the frequency f.,

- optical extraction means, the f_ part of the main radiation of the frequency to the input of the frequency shift organ depend _t

- an additional modulator that works at a low frequency

f, which differs from the frequency f ,. differentiates, is operated and which receives another part of the main radiation of the frequency f_,

- An additional optical system with which the radiation present at the output of the additional modulator is directed at the target is judged,

- a filter circuit, the input of which is connected to the output of the envelope detector is connected and has two outputs that are connected to the input of the low-frequency amplifier or are connected to the input of an additional low-frequency amplifier, which is connected to the additional modulator is,

- A divider circuit, the two inputs of which are connected to the output of the low-frequency amplifier or the additional one Connected to a low frequency amplifier,

- and includes a recording system connected to the output of the divider.

The invention is explained in more detail below with the aid of some exemplary embodiments with the aid of the drawings.

Fig. 1 shows schematically an embodiment of an inventive Contraption.

Figures 2A, 2B, 2C, 2D and 2E contain time diagrams of the Pulse amplitudes occurring in the device according to FIG. 1.

Fig. 3 shows schematically a further embodiment of a device according to the invention.

Fig. 4 shows schematically a remote detection system which is a Laser transceiver device according to FIG. 1 contains.

In Fig. 1 one sees a continuous laser transmitter 1, which is for example of the carbon gas waveguide type. This laser has two mirrors 3 and 4 along its axis 2, which form an optical resonance space. The mirror 3 is partially transparent, while the mirror 4 consists of a network, the inclination of which to the axis 2 is adjustable. Between the mirrors 3 and 4 there is a laser tube 5 which is delimited by transparent windows inclined in accordance with the Brewster angle of incidence. At the output of the transmitter 1 there is an optical separator lamella 6, which is partially reflected and inclined at 45 ° to the axis 2. Beyond the lamella 6 there are two converging lenses 9 and 10, which are centered on the axis 2 and form an afocal optical system. A low frequency modulator _; such as a shutter, lies in the common focal point of the two lenses. This closure has a disc 11 with openings and is set in rotation about its axis by a motor 12.

On the reflection axis 13 of the lamella 6 there is a germanium crystal 14 which is connected to an acousto-optical transmitter is in contact and is supplied by an electrical generator 15. A photoelectric detector 16 receives along an axis 17 the output radiation of the crystal. An afocal optical system with a converging lens 18 and

a diverging lens 19 is centered on an axis 20 which is the axis 17 between the crystal 14 and the detector 16 cuts. At the intersection of axes 17 and 20 there is a partially reflective optical lamella 21 in FIG inclined position such that it reflects the radiation which has passed through the afocal optical system 18-19. At the The output of the detector 16 is a processing circuit which successively includes an amplifier 22, an envelope detector 23 and an amplifier 24 connected in series.

A light generator 25 and a photoelectric receiver 26, which are arranged on a common carrier 27 and along one parallel to the axis 2, the openings of the disk. 11 arranged traversing axis, form a low-frequency modulation probe. The electrical output of the receiver 26 is connected to the amplifier 24.

The device according to FIG. 1 operates as follows:

When the motor 12 is switched on, the continuous radiation 28 of the frequency f ₁ , which originates from the transmitter 1, is transmitted through the disk 11 with its openings at a low frequency f 1. of a few kHz, for example, regularly interrupted. As a result, a pulsed beam 29 is formed at the exit of the lens 10 and is directed onto a target (not shown). A part 30 of the energy of these pulses is reflected by the target and, after passing through the optical system 18-19 and being reflected on the lamella 21, reaches the light-sensitive surface of the detector 16.

Due to the effect of the acousto-optical transducer assigned to the crystal 14, the part 31 of the radiation 28 which is reflected by the lamella is deflected by the crystal 14 and experiences a frequency shift f ~ -f ₁ , which can be of the order of 100 MHz. The beam 32 of frequency f

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which emerges from the crystal 14, forms a reference beam emitted by a local oscillator and illuminates the light-sensitive surface of the detector 16, to which, as mentioned, the echo radiation 30 of the frequency f .. is also fed. The detector 16 is of the square type and effects heterodyne detection; at its output it delivers a with the intermediate frequency t ^ -f. modulated electrical signal.

This signal is amplified by the intermediate frequency amplifier 22 and then passes through the envelope detector 23 one after the other and the low frequency amplifier 24, which provides synchronous detection at the low modulation frequency fr that of the rotating Closure is generated, means. For this purpose, the radiation generated by the generator 25 is periodically in accordance with the passage of the Openings in the disc 11 interrupted, and the amplifier 24 receives the electrical signals generated by detector 26 which are representative of the low modulation frequency and. This type of signal processing enables a significant reduction in the reception bandwidth and thus an increase 'the signal-to-noise ratio when receiving.

A parasitic back reflection of the laser impulses on the diopter surfaces the lens 10 passes through the lens and the lamella 6 in the opposite direction. After reflection on the output mirror 3 of the Laser 1, for example, this interference radiation is deflected along the axis 13 by the lamella 6. She then goes through the crystal 14 and experiences a frequency shift there

At the exit of the crystal 14, along the axis 17, a superimposition of the reference beam 32 with the frequency f 1 and the reflection which has the same frequency f 2 but is modulated with the low _{frequency f 1 of the rotating shutter is obtained.}

Pig. 2A shows the time dependence of the amplitude A of FIG this superposition resulting impulses, · namely considered at the exit of the crystal 14 in the event of parasitic reflection. .

Fig. 2B shows the shape of the echo pulses 30 of the frequency f., that were reflected by the target. These pulses are also modulated with the same lower frequency f_, but they have a delay that corresponds to the optical path to the target and back again, but this is practically negligible.

2C shows the shape of the electricity supplied by the receiver 16 '
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tric signal that modulates with the intermediate frequency t ^ -t

The amplifier 22 amplifies the intermediate frequency signal and practically blocks the low frequency. Figure 2D shows the shape the signals obtained at the output of this amplifier 22.

After passing through the detector 23, only the envelope curve of these signals remains, as FIG. 2E shows. The amplifier 24 brings about a synchronous detection of these low-frequency signals, which originate from the detector 26.

It can therefore be seen that parasitic reflection on the optical system on the transmitting side does not cause any noise to the received signals. This is essentially due to the fact that such a parasitic return radiation arrives at the receiver 16 with the frequency f ₀ after it has passed through the crystal 14.

In addition, the reference beam 32 at the exit of the crystal is a continuous beam and cannot give rise to a parasitic one Give back radiation at the exit surface of the crystal 14.

The device shown in Fig. 3 is preferably used when the amplitude of the rotating shutter

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introduced low frequency itiodulation is large. In this case it is very difficult to find an amplifier 22 which carries out the amplification of the intermediate frequency signals without saturation. The device shown in FIG. 3 uses the same elements as the device shown in FIG. 1 and thus also the same reference numerals. It also contains a polarizer 7, which is arranged on the axis 2 between the lamella 6 and the lens 9, and a quarter-wave lamella 8 ', which is also located on the axis 2 between the polarizer 7' and the lens 9.

The polarizer 7 subjects that passed by the lamella 6 Laser radiation of a straight or linear polarization in a first plane. This turns into a circular polarization converted by the quarter-wave lamella 8. There is therefore a parasitic reflection of the laser pulses on one of the diopter surfaces of the lens 10. This reflection 'passes through the lens 9 and the quarter-wave lamella 8 in the opposite direction, where the circular polarization is converted the radiation takes place in a linear polarization in a second Eftene perpendicular to the first plane. Any parasitic Reflection on the surfaces of the lens 10 is therefore blocked by polarizer 7.

In addition, as in the previous case, the laser beam deflected by the lamella 6, which the crystal 14 for generating the Reference beam passes through a continuous laser beam; a reflection of this beam on the faces of the crystal can also not cause an interference signal on the receiving side.

Fig. 4 shows a system by which a gas such as ozone in the atmosphere can be remotely detected.

This system contains a laser transceiver, which was explained with reference to FIG. In this device, the laser transmitter 1 generates radiation, the frequency of which is accurate

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corresponds to an absorption frequency of the gas to be discovered or detected, this frequency being achieved by rotating the network 4 can be set. The emerging from the optical system 10 beam 29 is partially when passing through a Layer of the sought gas absorbed.

This system also contains a further laser transmitter corresponding to the laser transmitter 1. The radiation 34 generated by the transmitter 33 has a frequency f _{which is close to the frequency f 1 of} the radiation emitted by the laser 1, but this frequency f does not correspond to an absorption frequency of the to discovering gas. The radiation 34 ″ runs along an axis 35 which is parallel to the axis 2 and successively passes through a partially reflective optical separator lamella

36 and an afocal optical system that has two converging lenses

37 and 38 has. At the common focal point of the lenses 37 and 38 is a rotating shutter, which is formed from a disk 39 resembling the disk 11 and around its axis is set in rotation by a motor 40. The disk 39 has openings which, when driven by the motor 40 cause a periodic blocking of the laser beam with a frequency f ^ which differs from the frequency fj. differs. A sequence of laser pulses is thus obtained at the exit of the lens 38 41, which are aligned in the same direction as the laser pulses 29. Ray 41 applies to that too discovering gas, but does not experience any absorption when this gas passes through it. Part of the energy of the ray 41 becomes partially reflected from the target along a path coincident with that of the beam 30. This reflected energy will from the receiver 16 after passing through the optical system 18-19 and receive reflection at lamella 21.

The separator lamella 36 reflects at right angles part of the energy of the beam 34 according to a beam 42, the the beam 31 after deflection at a mirror 43 and reflection at a partially reflecting and between the

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Lamella 6 and the crystal 14 lying optical lamella superimposed. The part of the energy of the beam 34 that has been deflected by the lamella 6 thus passes through the crystal 14 and experiences a frequency shift f ₄ ~ fU equal to the frequency shift f ₂ ~ f ..

The receiver 16 thus receives, on the one hand, two continuous reference beams from the crystal 14 with the frequencies f and f and, on the other hand, two beams which are formed by the returning pulses of the frequency f ₁ and f_ after deflection at the lamella 21.

The receiver 16 thus provides two types of electrical output at the output Signals, namely pulses of the frequency f_-f ■, the with the lower frequency f, - of the rotating shutter 11-12 are modulated, and pulses of a frequency f.-f, which from rotating shutter 39-40 with the low frequency f, modulated are.

The difference fn-f- is sufficiently large (of the order of magnitude

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of 10 Hz) to prevent the receiver 16 from the electrical Signals of the frequencies f -f or f ^ -f- outputs.

The electrical pulses of the frequency f ^ -fi and f -f, which are supplied by the receiver, first pass through the amplifier 22, which blocks the lower modulation frequencies, then the envelope curve detector 23, to finally enter a filter element 45, which is at an output 46 outputs the signals of the frequency f and at an output 47 the signals of the frequency f _fi.

The output 46 of the filter element 45 is connected to the input of the amplifier 24, which carries out _{a synchronous detection of the low-frequency signals' f 5 coming from the detector 26.}

The output 47 is connected to the input of a further amplifier 48, which enables synchronous detection on the low

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frequency f, of the rotating shutter 39-the shutter contains a frequency probe Shutter 11-12 is directed with a light generator beam onto a detector 50, wherein interrupted periodically according to the passage of the holes of the disc 3 9 through the beam path ^ electrical output of detector 50 is connected to 48.

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that of 9, which will η one of these beams rotating. The amplifier

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The outputs of amplifiers 24 and 48 are 51 and 52, respectively, of a divider circuit 53 Amplitude of the signal arriving at input 51 than that of the signals arriving at input 52 29 has an absorption in the to be discovered. The divider circuit 53 thus provides a of a recording device 54, a signal derived from the concentration of the gas with the length £ beam traversed layer is representative i | t

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two entrances closed. The e is less than the beam

learn the receipt for the product of the laser

Claims (1)

Fo 1 2766 D - - COMPAGNIE INDUSTRIAL DBS LASERS CILAS ALCATEL SJA. Route de'Nozay '91460 MARCOUSSIS ,, France LASER SEND RECEIVER with HETERODYNER detection ' PATENT CLAIMS .i ■ 1 J laser transceiver with heterodyne detection, which contains 2 - a laser transmitter that emits a continuous main radiation, τ -an optical separator that part of the radiation removes and lets the rest through, - a low-frequency modulator (Fr-.ci.ienz fL), the Separator receives the transmitted part of the main radiation and emits corresponding laser pulses, - an optical installed at the output of the modulator System that directs the laser pulses at the target, with part of the energy of these pulses corresponding to a Reflection of the frequency f, is backscattered, - a local oscillator ^ which emits a laser »reference radiation of a frequency f Φ f, - An electro-optical receiver for receiving the return radiation of the frequency f, and the reference radiation of the frequency f and for outputting an electrical response signal that is modulated with the intermediate frequency f., - r, - and a circuit for processing the electrical signal, characterized in that the local oscillator consists of a frequency shifting element (14) -that lies between the separator (6) and the receiver (16) and the part (31) of the main radiation (28) of the frequency removed from the separator (6) f receives, this element (14) delivering the reference radiation (32) of frequency f " _2. 2 - Device according to claim 1, characterized in that the processing circuit in Series at the output of the receiver (1.6) one after the other an amplifier (22) for the intermediate frequency signal, one Contains envelope detector (23) and a low frequency amplifier (24) which is connected to the modulator (11-12) and performs synchronous detection at the low frequency. 3 - Device according to claim 2, characterized in that g e k e η η zei chnet that the intermediate frequency signal amplifier (22) is able to block the low frequency. 4 - Device according to one of claims 1 to 2, characterized in that it has a polarizer (7) between the separator (6) and the modulator (11-12) in order to generate a rectilinear polarization of the part of the main radiation transmitted by the polarizer, and a quarter-wave plate (8) that: swischen the pole "riintor (7) and r> om modulator (" ■ -) to j-eor ^ ^ne: isi, ur :. di '• -jor?. 'dlinigo PsIprisation in a Zij-kulr.r ^ jl-risn'.ion umr; uwr ncle In. $ ■ - device according to claim 1, characterized in that the frequency shifting element (14) Οβ »OQ a crystal taken from the separator (6) Part (31) of the main radiation (28) is passed through, and an acousto-optical transmitter in contact with the Having crystal. 6 - Device according to claim 2 _S, characterized in that the modulator has a rotating Ver ^ closure, which in the common focus of one of the optical system (10) and an optical convergence element (9) between the closure (l; 1-! 2) and the separator (6) is arranged _{p is} formed afocal system. 7 - Device according to claim 6, characterized in that the connection between the low frequency amplifier (24) and the modulator (JL1-12) has an opto-electronic probe (25-26) which is attached to the rotating Shutter is assigned and electrical signals of the low-frequency modulation frequency f, - delivers, wherein the output of this probe with the input of the low frequency ·? · Amplifier (24) is connected. 8 - Device according to claim 1, characterized in that it comprises an optical lamella (21), which is in the path of the reference radiation (32) between the frequency shifting element (14) and the receiver (16) and which is aligned so that it sends the return radiation (30) to the receiver (16) and lets through the reference radiation (32). : 9 - Device according to claim 1 "characterized in that the laser transmitter (i) is a carbon gas laser of the waveguide type _{or the like} 10 - System for remote identification with a laser transceiver according to claim 2, characterized I guess it was also - an additional laser transmitter (33), which has a continuous Main radiation (34) of a frequency f which is shifted with respect to the frequency f. - Optical extraction means (36), which is a part (42) of the Direct the main radiation (34) of the frequency £ _ onto the input of the frequency shifting element (14), - an additional modulator (39-40), which is used in a Low frequency f ,, which differs from the frequency f ^, is operated and which receives the other part of the main radiation (34) supplied to the frequency f_, - An additional optical system (38) with which the at the output of the additional modulator. (39-40) existing Radiation is directed at the target, - A filter circuit (45), the input of which is connected to the output of the envelope detector (23) is connected and has two outputs (46-47) which are connected to the input of the Low frequency amplifier (24) or to the input of a additional low frequency amplifier (48) are connected, dea: connected to the additional modulator (39-40) is, - A divider circuit (53), the two inputs (51, 52) of which are connected to the output of the low-frequency amplifier (24) or of the additional low-frequency amplifier (48) are connected, - and a recording system (54) which is connected to the output of the Divider (53) is connected.